How many of your coworkers noticed the difference in image quality in portrait mode? The monitors are optimized for "landscape" and when you put them in "portrait" you get a very different color hue on each eye.
I found it so disturbing that I never used the monitors in portrait mode.
Not all panels have this problem. Of my monitors, only the one with the cheap TN panel has it (and it's so obvious it would drive me mad if I had to use it vertically). My screen with a PVA panel is fine.
It depends entirely on your panel and how well your operating system handles sub-pixel anti-aliasing. Some panels shift dramatically, and some operating systems do a terrible job and you'll notice all kinds of fringing.
Those panels from different lots have somewhat different colors anyway, even in the same orientation, and you can't tune them to be equal very easily, at least I gave up. They have pretty good viewing angles too. So it doesn't make it better or worse to turn either one 90 degrees.
Did you use a hardware calibrator? It shouldn't be hard to get a Dell monitor to a calibrated white point - they're not bad unless you got a TN panel model.
I found it so disturbing that I never used the monitors in portrait mode.